More configure options added to specify cflags/ldflags/libxxx/....

Signed-off-by: Xiongfei(Alex) Guo <xfguo@credosemi.com>
Update README.md to describe configure options and how to use.

Signed-off-by: Xiongfei(Alex) Guo <xfguo@credosemi.com>
Merge pull request #3 from xfguo/master

More configure options added to specify cflags/ldflags/libxxx/....; README.md updated.